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It is even worse than Geoff says. If you are starting off with a 'cold' camera that is you have just switched it on it and started a long exposure the sensor will heat up during this long exposure. The subsequent ICNR dark frame will then be to hot and leave black spots all over your image.
I bet you will find your first frame has these dark spots and the rest do not when doing sequential ICNR corrected exposures.
I measured the temperature of my Canon 5DH sensor before and after a long exposure. Within ten minutes it had risen by 17C degrees!
A C type sensor will have a smaller temperature rise at least 10+C.
Bert
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